drm/i915: HSW: allow PCH clock gating for suspend

For the device to enter D3 we should enable PCH clock gating.

v2:
- use HAS_PCH_LPT instead of IS_HASWELL (Ville, Paolo)
- rename lpt_allow_clock_gating to lpt_suspend_hw (Paolo)

Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
diff --git a/drivers/gpu/drm/i915/intel_display.c b/drivers/gpu/drm/i915/intel_display.c
index f5523a8..b890722 100644
--- a/drivers/gpu/drm/i915/intel_display.c
+++ b/drivers/gpu/drm/i915/intel_display.c
@@ -9271,6 +9271,11 @@
 	mutex_unlock(&dev->struct_mutex);
 }
 
+void intel_modeset_suspend_hw(struct drm_device *dev)
+{
+	intel_suspend_hw(dev);
+}
+
 void intel_modeset_init(struct drm_device *dev)
 {
 	struct drm_i915_private *dev_priv = dev->dev_private;